Function conditional_paths 

pub fn conditional_paths(
    source: &str,
    tokens: &[Token],
    dialect: &Dialect,
) -> Vec<ArmPath>

Record the preprocessor arm active at each token.

Directives never become clone content, but their nesting determines whether two otherwise-equal C-family snippets can coexist in one build. This pass intentionally recognises only literal 0 / 1 conditions; macro and expression evaluation belongs to a compiler frontend, not Fast mode.